1. Field of the Invention
The present invention relates to a connecting structure of a circuit substrate and, in particular, the present invention relates to a connecting structure for connecting flexible substrates and a circuit board of a liquid crystal display device and to a liquid crystal display having the same connecting structure. The present invention further relates to a mounting method of the liquid crystal display device.
2. Description of the Prior Art
A liquid crystal display has been widely used in a field of OA (Office Automation) devices, AV (Audio-video) devices and portable terminals, etc., due to its thin and lightweight structure and low power consumption characteristics thereof. The liquid crystal display device is generally constructed with a liquid crystal panel composed of a TFT substrate on which thin film transistors are provided as switching elements, an opposing substrate and liquid crystal sealed in a gap between the TFT substrate and the opposing substrate and an external substrate having external circuits such as drive circuit, etc., and connected to the liquid crystal panel, for controlling display thereon.
In order to connect the liquid crystal panel to the external substrate, it is usual to connect terminals on one side of at least one flexible substrate to a terminal portion provided on at least one peripheral edge portion of the liquid crystal panel and to connect terminals on the other side of the flexible substrate to terminals of the external substrate and, in order to realize the miniaturization and low cost of the liquid crystal display, an anisotropic conductive film (ACF) is usually used for connection between the flexible substrate and the external substrate.
The ACF is formed by dispersing electrically conductive particles in a thermosetting adhesive and, after the ACF is formed on one of the substrates of the liquid crystal panel by painting or the ACF in the form of a tape is adhered thereto and the other substrate is arranged in an opposing relation to the one substrate, the thermosetting adhesive of the ACF is solidified by heating it while the ACF is pressed to the connecting terminals with the conductive particles being in a gap between the connecting terminals of the respective substrates, so that it is possible to obtain high electric conductivity of the ACF while maintaining its mechanical strength, even in a case where the gap between the terminals is narrow and there is no margin in a mounting space.
The connecting method of the external substrate in the liquid crystal display device using the ACF will be described with reference to FIG. 1 and FIG. 7A to FIG. 11B. FIG. 1 is a perspective view showing a whole construction of the liquid crystal display device, FIG. 7A and FIG. 7B are plan views showing a structure of a conventional external substrate and FIG. 8 is a cross section taken along a line B-B′ in FIG. 7A. FIG. 9A to FIG. 9C are cross sections showing the steps of the conventional connecting method using the ACF, FIG. 10A and FIG. 10B and FIG. 11A and FIG. 11B illustrate problems of the conventional connecting method.
As shown in FIG. 1, a liquid crystal display device 1 comprises a liquid crystal panel 2 constructed with a TFT substrate having thin film transistors formed thereon as switching elements, an opposing substrate opposing to the TFT substrate and liquid crystal sealed in a gap between the substrates, a backlight 7 composed of a fluorescent lamp 10, a reflector 11 and a light guiding plate 9, etc., and casings 13a and 13b supporting these components. An external substrate having external circuits such as drive circuits formed thereon (referred to as a connecting substrate 4, hereinafter) is connected to a terminal portion 3 provided in at least one peripheral portion of one of the substrates of the liquid crystal panel 2. FIG. 7A shows the connecting substrate 4 in an enlarged scale and FIG. 7B shows a plurality of flexible substrates 5 connected to the connecting substrate 4.
A wiring pattern is formed on the connecting substrate 4 by using a copper thin film. The connecting substrate 4 is usually in the form of a lamination of a glass fiber cloth base, an epoxy resin layer and a copper film in the order and will be referred to as a glass-epoxy substrate material 4a, hereinafter. The connecting substrate 4 is electrically insulated and protected by a coating resin such as resist resin. In each connecting portion thereof to be connected to the flexible substrates 5, a non-coating portion 14 is provided by removing the coating resin provided thereon. In the non-coating portion 14, terminal group forming regions 15 are formed at a predetermined interval and a plurality of connecting terminals 4b are formed in each terminal group forming region 15 of the non-coating portion 14 by plating the patterned copper film with gold or nickel to form a terminal group. The terminal groups are connected to the flexible substrates 5, respectively.
FIG. 8 is a cross section taken along a line B-B′ of the connecting substrate 4 shown in FIG. 7A. As shown in FIG. 8, the terminal groups each having a plurality of the connecting terminals 4b arranged at a predetermined interval are formed on the glass epoxy base material 4a of the connecting substrate 4 as the copper thin film pattern. There is an exposed region 16a, in which the glass epoxy base material 4a is exposed, is provided between adjacent terminal group forming regions 15. Although it is generally practical that each connecting terminal 4b is several tens microns high and a pitch of the connecting terminals 4b is several hundreds microns, height of the connecting terminals 4b is shown exaggeratedly in FIG. 8.
The connecting method for connecting the connecting substrate 4 and the flexible substrates 5, which have the above mentioned structures, will be described with reference to FIG. 9A to FIG. 9C. It should be noted that, in the following description, the connection is performed by using an isotropic electrically conductive film (ACF). In general, the connection is performed by using the ACF 20 in two steps, that is, the first step of temporary press-bonding of the ACF 20 to one of the substrates of the liquid crystal panel at low temperature and the second step of final press-bonding of the ACF thereto at high temperature after the other substrate is arranged in an opposing relation to the one substrate to which the ACF is temporarily bonded.
In concrete, as shown in FIG. 9A, the ACF 20 is formed by uniformly dispersing electrically conductive particles in a highly insulating resin such as epoxy resin or acrylic resin and a separator film 19 is stuck on one of surfaces of the ACF 20. The ACF 20 with the separator 19 is cut such that it substantially coincides in size and shape with the non-coating portion 14 of the connecting substrate 4. The ACF 20 with the separator 19 is put on a surface of the non-coating portion 14 with the ACF 20 being in direct contact with the terminal forming regions 15 and the exposed regions 16a and the ACF 20 is temporarily bonded to the connecting substrate 4 by pressing the film down by a press-bonding head 18 while heating the lamination to a predetermined low temperature. This temporary press-bonding step is to temporarily bond the ACF 20 to the connecting substrate 4 with bonding strength enough to prevent the ACF 20 from being peeled off from the connecting terminals 4b thereof. In this state, the bonding resin of the ACF 20 is not completely solidified.
Thereafter, as shown in FIG. 9B, the separator 19 is peeled off from the surface of the ACF 20 to expose a bonding surface of the ACF 20. Thereafter, as shown in FIG. 9C, the flexible substrates 5 are arranged on the ACF 20 such that the flexible substrates are opposed to the respective terminal groups in the terminal forming regions and then the pressure is exerted on the flexible substrates 5 while heating them again to a predetermined high temperature. Therefore, the connecting terminals 4b of the connecting substrate 4 are electrically connected to the terminals of the flexible substrates 5 through the electrically conductive particles contained in the ACF 20 and the electrical connection is maintained fixedly by hardening the resin.
However, there are problems in the conventional connecting method for connecting the connecting substrate 4 and the flexible substrates 5 by means of the ACF.
That is, in the conventional method, the glass-epoxy base material 4a of the connecting substrate 4 is exposed in areas each between adjacent terminal forming regions and, therefore, there is a difference in height between the terminal forming regions and the exposed regions. The difference corresponds to the height of the connecting terminals 4b. Consequently, although temperature and pressure applied from the press-bonding head 18 are transmitted to the ACF 20 on the connecting terminals 4b substantially during the temporary press-bonding of the ACF 20 to the connecting substrate 4, the temperature and pressure are not transmitted enough from the press-bonding head 18 to the ACF 20 on the exposed regions 16a in which the glass-epoxy base material 4a is exposed. Therefore, bonding strength of the ACF 20 in the exposed region 16a is substantially low compared with that is the terminal forming regions 15 and the ACF 20 in the exposed regions 16a may be peeled off from the glass-epoxy base material 4a when the separator 19 is peeled off from the ACF 20, resulting in that the ACF 20 floats up in the exposed regions 16a as shown in FIG. 9B.
If the final press-bonding of the flexible substrates 5 is performed in such state, pressing force is transmitted from the floating portion 21 of the ACF 20 to the terminal forming regions 15 even if the connection is maintained in the temporary press-bonding, as shown in FIG. 9C, so that the reliability of connection is gradually degraded.
Further, depending upon the distance between adjacent terminal forming regions 15 and the surface condition of the exposed regions 16a, there may be a case where the ACF 20 in the exposed region 16a is attracted by the separator 19 and broken when the separator 19 is peeled off, as shown in FIG. 10A. If the flexible substrates 5 are mounted and the final press-bonding operation is performed in a state where the ACF 20 is broken and a broken portion of the ACF 20 is bent back as shown by reference numeral 22 in FIG. 10A, thickness of the ACF 20 is increased at location in which it is folded, so-that there may be a variation of the thickness of the ACF 20 on the connecting terminals 4b of the connecting substrate 4, as shown in FIG. 10B. Therefore, temperature thereof and pressure applied thereto are not uniform during the final press-bonding operation and so the bonding strength may be partially lowered and there may be portions 23, which lose an intimate electric contact. Further, in such case, since the ACF 20 is a resin film containing electrically conductive particles dispersed therein, the distribution of the electrically conductive particles on the respective connecting terminals 4b may become non-uniform, resulting in non-uniform electrical connection.
Further, there may be a case where the ACF 20 on not only the exposed regions 16a but also the connecting terminals 4b is turned up when the separator 19 is peeled off after the temporary press-bonding, as shown in FIG. 11A. In such case, there may be no ACF on a portion of the connecting terminals 4b. Therefore, it may be impossible to maintain normal electrical connection between the terminals of the flexible substrates 5 and the connecting terminals 4b as shown by a reference numeral 23 in FIG. 11B, resulting in that a resultant liquid crystal display device does not function normally.
It is clear that these problems result from the fact that it is impossible to absorb the step between the terminal forming regions 15 and the exposed regions 16a of the connecting substrate 4 caused by the lowness of temperature of the ACF 20 and pressure applied to the ACF 20 during the temporary press-boding step and degraded adhesion between them due to flatness of surface of the exposed regions 16a. As a method for relaxing the influence of such step between the terminal forming region and the exposed region, an insertion of a shock-absorbing member into between a press-bonding head and an ACF is proposed in, for example, JP H03-120790A.
In JP H03-120790A, during a temporary press-bonding step at low temperature or a final press-bonding step at high temperature, the shock-absorbing member is disposed between the press-bonding head and the ACF to absorb a difference in height between connecting terminal forming regions and exposed regions. The effect of the shock-absorbing member may be obtained if this technique is applied in the final press-bonding step in which temperature and pressure are high. However, when this technique is applied to the temporary press-bonding at low temperature, the effect of absorbing the step on a surface to which the ACF is bonded and uniformly transmitting temperature and pressure to the ACF can not be obtained enough due to lowness of temperature and pressure. Even if such effect is obtained, it may be varied depending upon the size of the exposed regions 16a. 
It may be possible to apply some pressure to the exposed region 16a between the terminal forming regions by providing the shock-absorbing member. However, since the glass-epoxy base material 4a is exposed in the exposed region 16a and the adhesive is not solidified enough in the temporary press-bonding, the adhesion between the ACF 20 and the glass-epoxy base material 4a becomes not enough, so that the problem of the peeling-off of the ACF 20 when the separator 19 is peeled off can not be solved.
Further, the shock-absorbing member disclosed in JP H03-120790A is gradually degraded and deformed by repetitive use thereof in the press-bonding step. Therefore, in order to reliably connect the flexible substrates 5 to the connecting substrate 4, the shock-absorbing members must be replaced frequently. Therefore, extra materials and additional fabrication steps are required, causing the cost of resultant liquid crystal display device to be increased.